On the Masking Pattern of a Simple Auditory Stimulus

THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA VOLUME 22, NUMBER 5 SEPTEMBER, 1950

On the Masking Pattern of a Simple Auditory Stimulus* JAMES P. EOAN AND HAROLD W. HAKE

University of Wisconsin, Madison, Wisconsin (Received April 15, 1950)

The masking audiogram of a pure tone is complicated by phenomena that arise from the interaction of the test tone with the masking stimulus. The production of beats and of difference tones results in a masking audiogram that does not represent the pattern of activity in the cochlea or nerve due to a simple masking stimulus. In the present experiments a narrow band of noise was used to mask pure tones. The "beat" heard in the immediate vicinity of the masking noise is not prominent, and a test tone higher in frequency than the band of noise is detected in terms of the characteristic pitch of that tone rather than by means of a difference tone.

With a band of noise slightly wider than a critical band (Fletcher), the amount by which the test signal at the center of the band exceeds the level/cycle of the noise is less than that obtained with a considerably wider band of noise. This smaller signal-to-noise ratio obtained with the narrow band of noise is probably due to "beats," since the test tone is heard as a "buzz" or "rattle." For purposes of deriving excitation or loudness patterns, a better estimate of the maximum amount of masking is obtained by using a wide band of noise whose pressure spectrum level is the same as that of the narrow band.

EASUREMENTS of the masking effects of an auditory stimulus provide data from which the excitation pattern of the masking stimulus may be de- rived. In 1924 Wegel and Lane • published the first quantitative results showing the masking of pure tones by pure tones. The masking audiograms which these 'investigators obtained showed how marked is the spread of action produced by a simple auditory stimulus.

The masking pattern of a pure tone is complicated, however, by various phenomena not directly related to the degree of excitation as it is measured by the masked stimulus. In the first place, when the masked stimulus, rs, is nearly the same in frequency as the masking stimulus, fp, beats occur, and the amount of masking is much less than at more distant points. Furthermore, the production of a difference tone, fs-fp, by the distortion of the masking and masked stimuli, makes it possible to detect the presence of the masked stimulus long before its intensity is sufficient to provide its characteristic pitch. When the difference tone is detected, the amount of masking measured at f• depends upon the amount of distortion in the ear under test and upon the amount of masking at the frequency (f•-fv) of the difference tone. Clearly, under these conditions, the degree of excitation at the frequency f• is not adequately measured by the amount of masking at f•. Fletcher and Munson 2 were fairly suc- cessful in avoiding these difficulties by using bands of noise as the masking stimulus. With this stimulus the effects on masking due to beats and to difference tones are largely, but not entirely, obviated. The measurements reported below provide further support for the

* This research was supported in part by the Research Com- mittee of the Graduate School of the University of Wisconsin from special funds voted by the State Legislature.

• R. L. Wegel and C. E. Lane, "The auditory masking of one pure tone by another and its probable relation to the dynamics of the inner ear," Phys. Rev. 23, 266-285 (1924).

2 H. Fletcher and W. A. Munson, "Relation between loudness and masking," J. Acous. Soc. Am. 9, 1-10 (1937).

use of narrow bands of noise in the determination of the

masking pattern of a simple auditory stimulus?

EXPERIMENTAL PROCEDURE

The masking audiograms of two stimuli were determined. One masking stimulus was a pure tone of 400 c.p.s., generated by a beat frequency oscillator (General Radio Co., Type 1304-A). To ensure the purity of this tone it was passed through the same narrow band-pass filters used for the other masking stimulus. The second masking stimulus was a narrow band of noise obtained by passing a noise (gas tube) with uniform spectrum level through narrow band-pass filters (two wave filters, General Radio Co., Type 530-A). The resulting band of noise was about 90 c.p.s. wide, measured three decibels down from the broad peak, and had a center frequency of 410 c.p.s. The over-all voltage level of this band of noise was measured directly and also checked by calculation from the over-all voltage measurements of a wider band of noise (0-1000 c.p.s.) and the measured filter characteristics. Since the noise had a unit•orm spectrum level, the level-per-cycle was taken to be -10 log•0Af decibels relative to the over-all level, where Af is the band width of the noise whose over-all level is known. The voltage levels measured at the earphone were converted to sound-pressure levels (S.P.L. or decibels re 0.0002 dyne/cm 2) by means of the calibration curve of the earphone.

All measurements were monaural, and the masking and masked stimuli were presented by means of a dy-

a A recent article by W. A. Munson and M. B. Gardner, "Loud- ness patterns--a new approach," J. Acous. Soc. Am. 22, 177-190 (1950), points out the difficulties encountered in the determination of the excitation or loudness pattern of a pure tone. These investigators employed a novel technique in which a test tone of brief duration is presented a short time after the masking stimulus is turned off. By this method they obtained measures of residual masking which are evidently free of the complications due to beats, difference tones, and aural harmonics. Their important new findings are not related to the amount of simultaneous masking in a simple manner, however.

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THE MASKING PATTERN 623

namic earphone (Permoflux Corp., Type PDR-10) held in a headband and coupled to the ear by means of a small rubber cushion. Under these conditions the sound

pressure at 400 c.p.s. near the entrance to the auditory canal does not differ appreciably from that obtained with the artificial ear (6 cc coupler) used for calibration of the earphone. 4 The contralateral ear was also covered by means of a PDR-10 earphone held in the headband and sealed against the external ear by means of a small rubber cushion. The internal consistency of the results obtained with low and high intensities of the masking stimulus as well as independent checks (the masking of the contralateral ear by white noise and the auditory localization of the masked tone near its masked thresh-

old) indicate that any cross-stimulation of the contralateral ear did not appreciably affect the results.

Once the sound-pressure level of the masking stimulus is correctly measured, discrepancies between the actual pressure of the masked stimulus and that found with the artificial ear do not affect the determination of

masking, since a difference between two intensities at the same frequency is the required datum. Conse-

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quently, the marked low frequency leak (below 300 c.p.s.), which is present when the earphone is worn, does not introduce errors in the masking results.

The masking audiograms for each of these masking stimuli were determined at various sound-pressure levels. The number of intensities used varied somewhat

from listener to listener, and the particular intensities will be indicated along with the results.

In all, seven subjects served in the experiment. Two of these Ss had marked hearing losses at low frequencies, and their results are not included in the present report. Each of the other five Ss had normal hearing. The curve showing the average absolute threshold as a function of frequency for these five Ss agrees remarkably well with that published by Sivian and White 5 for minimum audible pressures with monaural listening. At 100 c.p.s. the deviation from Sivian and White is 4 db. From 200

to 5000 c.p.s. the deviation at any frequency is less than 2 db.

The amount of masking was measured in the usual way. The absolute threshold in quiet for pure tones was first determined as a function of frequency. Four-

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Fro. 1. The curves are masking audiograms of a pure tone of 400 c.p.s. The number under each curve is the sound-pressure level (decibels re 0.0002 dyne/cm •') of the masking tone (0 db sensation level corresponds to 15 db S.?.L. at 400 c.p.s.). The abscissa represents the frequency oœ the masked tone and the ordinate represents the shift in decibels of its absolute threshold. Note how asym- metrical the curves become as the intensity of the masking stimulus is increased.

a F. M. Wiener and A. S. Filler, "The response of certain earphones on the ear and on closed couplers," in a report of research issued by the ?sycho-Acoustic Laboratory, Harvard University, under a contract with the U.S. Navy, Office of Naval Research, Contract N5ori-76, Project order II, Report ?NR-2, December 1, 1945.

5 L. J. Sivian and S. D. White, "On minimum audible sound fields," J. Acous. Soc. Am. 4, 288-321 (1933).


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624 J. P. EGAN AND H. W. HAKE

teen tones ranging from 100 to 8000 c.p.s. were used for this purpose. At least two measurements were made at each frequency by each subject. Then the thresholds were redetermined in the presence of the masking stimulus. The 29 pure tones used as the masked stimuli, rs, were: 100, 200, 300, 340, 370, 390, 397, 403, 410, 430, 460, 500, 600, 700, 797, 803, 900, 1000, 1197, 1203, 1400, 1597, 2000, 2400, 2800, 3200, 4000, 5000, and 6000 c.p.s. Another oscillator (General Radio Company, Type 1304-A) was used to generate the masked stimulus, rs. When the masking stimulus was a 400 c.p.s. pure tone, special care was exercised to ensure that the relative calibration of the two oscillators, one for the masking and one for the masked stimulus, was correct. Thus, when the masked stimulus was near 400 c.p.s., the experimenter first adjusted the frequency of the oscillator used for the masked stimulus until it gave zero beat against the oscillator used for the masking stimulus (400 c.p.s.). Then the incremental tuning dial, calibra ted in one-cycle steps, was set to give the appro- priate frequency for the masked stimulus. The same procedure was followed when the frequency of the masked stimulus differed frbm an aural harmonic by 3 c.p.s. In this case the masked stimulus could be made to beat

clearly against the aural harmonic of the pure masking stimulus, and zero beat was easily obtained. Because a large number of frequencies were tested, and because the repeated calibration of the oscillators held up the masking tests, only one determination of the masked threshold by each subject was feasible.

For the determination of both quiet and masked absolute thresholds the test tone, f•, was presented auto- matically by means of an electronic switch which turned the tone on and off at regular intervals. The duration of the test tone was 0.7 sec. After an interval of 0.7

sec. the test tone sounded again. The effects of transi- ents were minimized by turning the tone on or off over a period of about 0.1 sec.

In all tests the subject adjusted an attenuation net- work, calibrated in one decibel steps, until he found his own threshold. He was carefully instructed in the procedure of "bracketing" his threshold. When such a method is carefully employed, the datum from a single trial is actually the result of a series of judgments made by the subject before he selects the final setting. When the test tone was at or near threshold, the subject could effectively turn off the masked tone by throwing 10 db additional attenuation in the pad, an aid which some subjects find useful.

In determining his masked threshold the subject used the criterion "detection of anything." Thus, with a masking stimulus of 400 c.p.s. and a masked stimulus of 300 c.p.s., he would hear, in addition to the masking tone, the characteristic pitch of a 300 c.p.s. tone. At 397, however, he would hear nearly two beats while the test tone was on and then a short interval during which the masking stimulus sounded without waver. Under these conditions his task was to adjust the

attenuator dial until he could just detect the presence of the beat. Again, when a tone of 600 c.p.s. was near the threshold of detection (masked threshold) in the presence of a loud tone of 400 c.p.s., he would typically hear a tone whose pitch corresponded to a tone of 200 c.p.s. (f•-f•). For all masked thresholds, he adjusted the attenuator until he could just detect that something happened at the regular intervals during which the test tone f• was on.

RESULTS AND DISCUSSION

Typical masking audiograms of a pure tone of 400 c.p.s. are shown in Fig. 1. The number under each curve designates the sound-pressure level of the masking stimulus. As was found by Wegel and Lane, • the ma•k- ing audiogram of a stimulus of low intensity is nearly symmetrical when frequency is plotted on a logarithmic scale. However, at the higher intensities of the masking stimulus, the aural harmonics of the pure masking tone help to spread the action of the primary stimulus up the frequency scale. The curves also show the well-known "dips" due to beats between the masked stimulus and the masking stimulus or its aural harmonics. When the frequency of the masked stimulus is 397 or 403 c.p.s., the amount of masking is evidently determined by the value of the differential threshold for intensity at 400 c.p.s. As the frequency of the masked stimulus increases up to 430 c.p.s., the amount of masking increases and passes through a maximum. Above about 430 c.p.s. the amount of masking decreases because presumably the ear is now successfully analyzing the complex tone into its two components. The masked tone is then heard as a separate component. At the higher intensities of the masking stimulus, the masking curve passes through a minimum between 400 and 800 c.p.s. The curve drops as much as it does because the difference tone, f•-f•, is detectable before the masked stimulus is heard with its characteristic pitch.

Figure 2 shows the masking audiograms for the same subject when the masking stimulus is a narrow band of noise. It is evident that the masking pattern is not nearly as complicated by beats, harmonics, and difference tones as is the masking pattern of a pure tone. The "beat" heard in the immediate vicinity of 410 c.p.s. is no longer prominent, and the test tone, fs, is heard as a "buzz" or "rattle." Furthermore, when the over-all S.P.L. of the band of noise is 80 db, the test tones of 797 and 803 c.p.s. appear with only a slight "flutter" or "roughness." Careful listening tests also indicate that a masked tone of 500 or 600 c.p.s. is detected in terms of that tone and not by means of a difference tone.

A comparison of Figs. 1 and 2 •hows that, at the lower intensities of the masking stimulus, the masking audiogram of the pure tone lies wholly under the corresponding masking audiogram of the band of noise. With the masking stimulus at an over-all S.P.L. of 80


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db, however, the pure tone of 400 c.p.s. masks the high frequencies better than the band of noise.

The relation between the amount of masking at a certain frequency and the intensity of the masking stimulus is better illustrated by Figs. 3 and 4. Figure 3 shows the amount of masking at a given frequency as a function of the intensity of the masking tone of 400 c.p.s. The curve which shows the amount of masking at 430 c.p.s. has a slope of one only at intermediate intensities of the masking stimulus; at higher intensities the curve departs more and more from linearity. The curves for 1000 and 1400 c.p.s. show how great is the increase of masking at frequencies just above the second and third aural harmonics of the pure masking stimulus. These curves ultimately cross the curve for 430 c.p.s., a fact which verifies the result obtained by Wegel and Lane. •

In Fig. 4 are shown the corresponding curves of masking produced by the narrow band of noise. For this masking stimulus the curve that shows the amount of masking at 430 c.p.s. is a linear function with a slope of one over most of its course. The equation of this curve over its linear portion is: M=10 log•o(I,,,Afk/ loaf,,,), where M is the amount of masking in decibels, I,• is the over-all intensity of the masking noise, Af,• is the band width of the masking noise in c.p.s., Afk is

the width of a critical band in c.p.s., and I0 is the threshold intensity of the masked tone measured in the quiet. Now, 10 log•0Af,• is 19.5 db; 10 log•0Af• obtained by using the narrow band of noise is 14.5 db for this subject; and his absolute threshold is 14 db S.P.L. at 430 c.p.s. Therefore, the simplified equation of the curve is M=/•-19, where/• is the band pressure level (decibels re 0.0002 dyne/cm 2) of the masking stimulus. (The fact that k is only 14.5 db is discussed below.)

Figures 5 and 6 show for a different subject the same set of relations as those of Figs. ! and 2. This subject (HH) was also used to obtain data comparable to those of Figs. 3 and 4, but they are not included because they are so similar to the data presented.

Figure 7 shows the average data for five subjects (JE, LG, HH, LS, and MS). The individual data of two of the Ss (JE and HH) have been presented. The other three Ss show essentially the same relations between the two masking audiograms as those shown by the average curves. With a masking stimulus at an over-all sound-pressure level of 80 db, four of the five Ss show that tones above about 1000 c.p.s. are masked more by a pure tone of 400 c.p.s. than by the narrow band of noise. The intensity of the masking stimulus at which an individual's masking audiogram for a pure tone of 400 c.p.s. crosses the audiogram for a band of noise

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Fro. 2. These curves are masking audiograms of a band of noise (90 c.p.s. wide, centered at 410 c.p.s.). The parameter is the over-all sound-pressure level of the masking noise. These curves represent better than those for a pure tone the masking patterns of an auditory stimulus whose energy is confined to a narrow frequency region; but the masking at 410 c.p.s. still does not have the same meaning as the masking at more distant points, because the masked tone at about 410 c.p.s. is detected as a "buzz" or rattle"


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Fro. 3. Each curve shows the amount of masking at a given frequency as a function of the intensity of the masking tone of 400 c.p.s. Note how the curve for the masked tone of

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Fro. 5. Masking audiograms of a pure tone of 400 c.p.s. These curves show for a different subject the same relations as those of Fig. 1.

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indicates the degree of distortion in the ear of the individual. It is fairly certain that at higher intensities all subjects with normal absolute sensitivity curves would show the crossing of the two masking audiograms.

The variance of the data for the five Ss obtained with

the band of noise was analyzed into various sources. Inspection of the data clearly revealed that the variability of the masked threshold for a given tone depends to a marked degree upon the frequency of that tone. Accordingly, the data were subdivided into three sets. One set consisted of the masked thresholds for tones

whose frequencies were below that of the masking stimulus. Those tones whose frequencies were in the same region as the frequency of the masking stimulus comprised the second set, and the tones higher in frequency than that of the masking stimulus composed the third set. The total number of judgments in each set was 20, 35, and 35, respectively. The measures of variability obtained by these analyses are shown in Table I. The column headed "S.D. of judgments by an average S" may be thought of as the standard deviation of a distribution of a large number of thresholds for a single masked tone measured with an average listener. Such a distribution provides a measure of the inherent

variability or of the error of measurement that remains after differences due to the frequency of the masked tone and individual differences among the listeners have been removed from the total variance of the data.

The standard deviation of the distribution of means, one for each listener, is a measure of differences among listeners. (The differences among the listeners were not significant for the four tones of low frequency.)

It is clear from the data of Table I that the vari-

ability of the masked thresholds is relatively small when the energy of the masked stimulus is concentrated in the same band of frequencies as that of the masking stimulus. However, when the masking is the result of "spread" of action of the masking stimulus, the variability is relatively large, particularly so when the spread is up the frequency scale.

It was not considered meaningful to perform a similar analysis of variance for the data obtained with the pure masking tone, since the assumption of homo- geneity of variance for the frequencies grouped into one set could not be met. Although the criterion used in the determination of the masked threshold was the

same throughout, the basis for the detection of the masked tone varies markedly as pointed out above.

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Fro. 7. These two curves show the difference between the masking audiograms of a pure tone of 400 c.p.s. and of a narrow band o f noise (90 c.p.s. wide, centered at 410 c.p.s.) with each presented at an over-all sound-pressure level of 80 db (decibels re 0.0002 dyne/ cm•'). Comparison of the two curves shows how much the masking pattern of a pure tone is complicated by beats, aural harmonics, and difference tones. The dips are due to beats, and the broad minimum between 400 and 800 c.p.s. is due in part to the detection of the masked tone by means of the difference tone and in part to the additional masking produced by the aural harmonic of 800 c.p.s. These masking audiograms are based upon the average of 5 Ss with normal hearing.


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pressure levels (decibels re 0.0002 dyne/cm2). The pressure spectrum level of the noise may be obtained by subtracting 19.5 db from the corresponding number under the curve. The peak of each masking curve is extended by 4.2 db in order to represent better the amount of excitation near the frequency of the masking stimulus.

The average masking audiogram of a tone of 400 c.p.s. (Fig. 7) was compared with the corresponding data published by Wegel and Lane2 With masked stimuli below 400 c.p.s. the agreement is excellent. However, at 450 and 1000 c.p.s. Wegel and Lane found much more masking (by 10-15 db) than that measured in the present experiment. Above 2000 c.p.s. the dis- agreement is not as marked. Since there are large differences among listeners, it is advisable that the original experiment of Wegel and Lane, which covered such an extensive range of intensities and frequencies of the masking tones, be repeated with several normal ears.

Incidentally, it should be recorded that the value of /XE/l•. obtained with the five Ss agrees well with that found by Riesz ø for the same conditions of measurement. When a pure tone of 400 c.p.s. is the masking stimulus, the values of the masked threshold at 397 and 403 c.p.s. provide one type of measure of the differential threshold for intensity. The value of AE/E found in the present experiment for a 400 c.p.s tone at a sensation level of 65 db is 0.19.

The degree to which a band of noise masks a tone whose frequency is near the center of the band is re-

6 R. R. Riesz, "Differential intensity sensitivity of the ear for pure tones," Phys. Rev. 31, 867-875 (1928).

lated to two principal variables: the intensity of the noise and the width of the critical band. If the masking noise is at least as wide as a critical band, then the value of the masked threshold should be given by the following expression' MT=B+k, where MT is the masked threshold, B is the intensity/cycle, and k is the width of the critical band. The three values are

expressed in decibels with the same reference intensity for MT and B. The maximum value of the average masked threshold for the five Ss was 74.5 db (S.P.L.), and the pressure spectrum level of the noise was 60.5 rib. In the present experiment the value of k thus turns out to be 14 db. Now, Fletcher 7 as well as Hawkins and Stevens 8 have found that with a wide band of

noise k is 17 db at 410 c.p.s., and the difference of 3 db between the values of k found with a narrow and

with a wide band of noise is too large to explain away in terms of individual differences or errors of measurement. This difference in the value of k must be at-

tributed to the fact that even with a band of noise,

7 H. Fletcher, "Auditory patterns," Rev. Mod. Phys. 12, 47-65 (1940).

• J. E. Hawkins, Jr. and S.S. Stevens, "The masking of pure tones and of speech by white noise," J. Acous. Soc. Am. 22, 6-13 (1950).


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TABLE I. Standard deviations of masked thresholds of tones heard in the presence of a band of noise centered at 410 c.p.s.

S.D. of judgments by S.D. of means Frequencies of masked tones an average S of $s

100, 200, 300, 340 2.9 db 1.2 db

370, 390, 397, 403, 1.3 1.8 410, 430, 460

900, 1000, 1197, 1400, 3.6 7.1 1597, 2000, 2400

the masked tone "beats" with the randomly fluctuating stimulus. Since all subjects reported that the test tone was heard near the masked threshold as a "buzz" or "rattle" it is almost certain that the value of k is underestimated when the masking noise is confined to a narrow band of frequencies. In view of these con- siderations the value of k at 410 c.p.s. was redetermined using the same five Ss and a wider band of noise (0- 1000 c.p.s.). This noise had the same pressure spectrum level as the narrower band of noise (90 c.p.s.). The signal-to-noise ratio at the masked threshold was found to be 18.2 db. With the wider band of noise the sub-

jects could not hear the masked tone until it was 4.2 db more intense than that required for the narrow band. It is important to note that with the wider band of noise the subjects do not hear a "buzz" or "rattle" when the masked tone is just above the masked threshold.

It is clear that when masking is used as the measure of excitation the value of k obtained with the wider band of noise is a better estimate than that obtained with the narrow band. Therefore, the masking audiograms shown in Fig. 8 were constructed. The solid circles come from Fig. 2 and the open circles from Fig. 4. The unbroken curves are merely a visual fit to the data with an attempt to maintain a spacing of 10 db between the maxima of the curves. Since the amount of masking near the center of the band of noise is too small, the peak of each masking audiogram was extended by 4.2 db in order to represent better the amount of excitation near the frequency of the masking stimulus. This cor- rection probably makes it possible to interpret the amount of masking near the masking stimulus in the same terms as that measured at distant points along the frequency scale.


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On the Masking Pattern of a Simple Auditory Stimulus